State-of-the-art cardiac defibrillators are powered by lithium batteries in conjunction with electrolytic capacitors. The batteries contain a metallic lithium negative electrode, a silver-vanadium oxide positive electrode and a non-aqueous liquid electrolyte consisting of a lithium salt such as LiAsF6 dissolved in an organic solvent, such as propylene carbonate. Silver vanadium oxide electrodes are well known in general for lithium batteries, as described in U.S. Pat. No. 4,310,609 and U.S. Pat. No. 4,391,729 that disclose the use of an electrochemical cell having as its positive electrode a composite oxide matrix consisting of a vanadium oxide chemically reacted with a group IB, IIB, IIIB, IVB, VB, VIIB, VIIB and VIII metal, and most specifically with a silver containing compound. U.S. Pat. No. 4,391,729 also discloses a method of making such a cathode. The current positive electrode of choice is Ag2V4O11. Li/Ag2V4O11 cells discharge by an electrochemical process that involves lithium insertion into the crystal lattice of Ag2V4O11 with a simultaneous reduction of the silver ions and their concomitant extrusion from the crystal lattice; thereafter, lithium insertion is accompanied by a concomitant reduction of the vanadium ions within the structure, ideally from V5+ to V4+. Thus the reaction can be broadly defined in the ideal case as taking place in main two steps:Li+Ag2V4O11→Li2V4O11+2Ag  (Step 1: Silver displacement)xLi+Li2V4O11→Li2+xV4O11 (xmax≈5)  (Step 2: Lithium insertion)
One of the major limitations of Li/Ag2V4O11 cells is that they lose their capability of providing the necessary power particularly after the reaction described in Step 1 has occurred, and when cells are allowed to stand for prolonged periods of time. It is believed that this loss in power may be attributed, at least in part, to the Ag2V4O11 positive electrode, and in particular, that it may be attributed to the fact that at the end of Step 1, a metastable phase of composition Li2V4O11 is formed. This metastability is supported by the fact that it has not been possible to synthesize a Li2V4O11 phase that is isostructural with Ag2V4O11 by independent chemical methods in the laboratory. Attempts to synthesize a “Li2V4O11” phase in the laboratory, for example, by reacting Ag2V4O11 with n-butyllithium, have failed; these attempts have always yielded other stable lithium-vanadium-oxide phases such as LiVO3 and LiV3O8. This finding indicates that the power fade may at least be partly attributed to a decay of the metastable “Li2V4O11” phase that is generated electrochemically during Step 1 into other more stable lithium-vanadium-oxide compounds.
Li/Ag2V4O11 lithium cells deteriorate prematurely and are unable to deliver acceptable pulse power before the cells have reached the end of their expected calendar (shelf) and operating life. It is understood that such limitations of Li/Ag2V4O11 cells are of great concern when used to power implantable devices such as cardiac defibrillators in the human body. Such limitations negatively affect product reliability and necessitate a continual monitoring of the cells while implanted in patients to ensure a timely replacement before they prematurely reach the end of discharge.
There is therefore a great need to improve the electrochemical properties and operating life of silver-vanadium-oxide electrodes for lithium cells and batteries, particularly for use in life-supporting medical devices. In this respect, silver manganese oxide AgxMnOy electrodes derived from silver permanganate precursors have already been fabricated and disclosed in U.S. Pat. No. 7,041,414, the entire disclosure of which is incorporated by reference. We have now discovered that by using a combination of manganese and vanadium together with the silver component of the metal oxide electrodes, it is possible to significantly increase the capacity of the electrodes and hence the energy density of the lithium cells and batteries in which they are employed.